1. Field of the Invention
The present invention relates to a jitter measuring apparatus and a measuring method for measuring jitter in a signal-under-measurement and a test apparatus for testing a device-under-test.
2. Related Art
Conventionally, as a method for measuring jitter in a signal-under-measurement, there has been known a method of comparing phase of the signal-under-measurement with phase of the signal-under-measurement delayed by one period for example (T. Yamaguchi, “A Real-Time Jitter Measurement Board for High Performance Computer and Communication System”, ITC2004). Period jitter in the signal-under-measurement may be measured by delaying the signal-under-measurement by one period and by comparing phases of neighboring rising edges in the signal-under-measurement.
According to this method, the signal-under-measurement and the delayed signal-under-measurement are applied to a phase comparator so as to output a voltage signal corresponding to a value of the period jitter in each cycle. Timing jitter in the signal-under-measurement may be also measured on real-time by integrating the voltage signal outputted out of the phase comparator by using an integrator or the like.
However, the conventional jitter measuring method has required to delay the signal-under-measurement accurately. For example, it is necessary to delay the signal-under-measurement by one period accurately in measuring the period jitter in the signal-under-measurement. However, if there is an error in the delay time, the value of the period jitter detected by the phase comparator becomes a value into which the actual period jitter is added by the delayed error as an offset. Still more, the measured error is accumulated by the integrator and timing jitter outputted out of the integrator saturates soon or later.
Accordingly, the conventional jitter measuring method has required a delay circuit that generates accurate delay time. However, it has been difficult to produce an accurate delay circuit and it has been difficult to measure jitter in the signal-under-measurement accurately by the conventional jitter measuring method because the delay error is influential in the jitter measuring error.
Still more, the phase of the signal-under-measurement is also compared with phase of the delayed signal in the conventional jitter measuring method. Therefore, when jitter amplitude in the signal-under-measurement is greater than one period of the signal-under-measurement, there has been a possibility that phases of non-corresponding edges are compared and hence the jitter could not be accurately measured.
It was also unable to accurately measure timing jitter in a signal in which logical values appear randomly such as a data signal by the conventional jitter measuring method. For example, it is necessary to generate a triggering signal for measuring phase of edges of the data signal-under measurement in measuring timing jitter by using one of conventional methods, i.e. by using a sampling oscilloscope, a digital oscilloscope and the like. However, because position of the edges is irregular in the data signal, it is necessary to generate triggering signals of various timing. Therefore, timing jitter of the triggering signals themselves becomes large, worsening the accuracy in measuring jitter in the data-signal-under-measurement.
A measuring cost also increases to generate triggering signals having small timing jitter because a high precision circuit is required.
Therefore, it was unable to measure timing jitter in the data signal whose pattern is irregular at low cost and accurately by the conventional measuring methods. Still more, neither method nor circuit for measuring jitter in the data signal on real-time has been invented yet.
Accordingly, it is an object of the invention to provide a jitter measuring apparatus, a jitter measuring method and a test apparatus that are capable of solving the above-mentioned problems. This object may be achieved through the combination of features described in independent claims of the invention. Dependent claims thereof specify preferable embodiments of the invention.